Terahertz (THz) radiation, with frequencies ranging from 0.1 to 10 THz, bridges the gap between radio frequencies and the infrared. This part of the electromagnetic spectrum remains the least explored, mainly due to technical difficulties involved in making efficient and compact THz sources and detectors. The lack of suitable technologies led to the THz band being called the “THz gap”. This technological gap is rapidly diminishing with tremendous advances in optical technologies in the last two decades.
THz sources and detectors have many important potential applications in the areas of spectroscopy, detection, and security [1-3]. For example, the low energy photons of the THz radiation can be safely used on humans for security purposes at airports, to detect weapons and explosives. THz radiation can also be used to study the vibrational modes of several biological and chemical molecules, since these modes have energies corresponding to THz photon energies. One example is the twisting and deformation of the double-helix structure in DNA. THz radiation is also suitable to study carrier transport and relaxation dynamics in condensed matter, such as bulk semiconductors, semiconductor quantum wells, carbon nanotubes, semiconductor quantum wells, and single-monolayer graphene sheets.
Various methods such as photoconductive antenna [4], electro-optic (EO) sampling [5], and air-biased-coherent-detection (ABCD) [6] have been proposed and demonstrated in order to measure the spatial and temporal profile of the THz electric field.
Among these, the electro-optic (EO) sampling method is widely used for THz time-domain spectroscopy (THz-TDS), due to the relatively straightforward physics involved and its wide bandwidth [7].
In the electro-optic (EO) sampling method, a linearly polarized femtosecond laser pulse co-propagates with a few picosecond THz pulse in an electro-optic (EO) crystal. The THz electric field induces birefringence in the crystal, which changes the polarization of the linearly co-propagating laser pulse. The change in the phase between the two polarization components of the probe beam, which is proportional to the THz electric field, is measured by using two cross polarizers before and after the detector crystal, and appears as a modulation in the intensity of the probe beam. The complete THz waveform is then reconstructed by scanning the probe pulse over the entire THz pulse.
Several improvements over the EO sampling method have been proposed in the past to improve the detection of THz radiation. Such methods include a chirped-pulse or spectral-encoding method [8], a cross-correlation method [9], a two dimensional THz pulse characterization method with dual echelons [10], and a tilted wavefront detection method using prism [11].
Since a major advantage of THz radiation is the possibility of coherent detection, in which information on both the electric field and phase can be measured simultaneously, THz radiation is a very important tool for spectroscopy and imaging applications. In order to make use of advantageous features, such as wide bandwidth, of THz sources, a detection technique with (i) high spectral resolution and (ii) high sensitivity is needed.
The spectral resolution (Δμ) obtained from a temporal scan of THz pulse depends on the length (T) of the scan (Δμ=1/T). Therefore, to realize higher spectral resolution in THz-TDS, one needs to take waveform scans over longer time. In order to allow longer time scans, one needs to use thicker detection crystals. This is because part of the femtosecond probe beam is reflected from the two faces of the crystal. If a thin crystal is used, these internal reflections interfere with the main detected THz signal, which induces beating in the spectrum, thus distorting the measurement.
High sensitivity is achieved with the use of a lock-in amplifier. In order to use the lock-in amplifier, first the THz signal is chopped at f/2 frequency where f is the repetition rate of the laser and then this signal is fed to the lock-in amplifier, which filters the THz signal, increasing its sensitivity.
All methods up to now, which are based on EO sampling, use two cross polarizers, i.e. a first one placed before and a second one placed after, the detector crystal, to measure the polarization rotation of the probe beam, from which THz electric field is evaluated. However, when the polarization of the probe beam rotates more than 90°, a reversal in the intensity modulation of the detection beam occurs, thus not allowing the correct measurement of the THz electric field and waveform [12]. This situation is referred to as “over-rotation”. The birefringence introduced in the EO crystal is proportional to the THz electric field and the thickness of the crystal. Thus the issue of over-rotation increases with the increasing thickness of the crystal. One could in principle use thinner crystals to avoid over-rotation, but the use of thin crystal causes interference effects discussed above. With the recent advances in high power THz generation methods, over-rotation is becoming a major issue for THz-TDS.
On the other hand, thinner crystals result in reduced THz signal and thus poor signal-to-noise ratio (SNR) of the measurement, due to a decrease in the interaction length. In a previously proposed ABCD method [6], although not limited by the problem of over-rotation, the use of a high voltage supply and lock-in amplifier makes it more complicated when compared to the EO sampling method.
There is still a need for a THz electric field measurement method and system.